As one of the wireless communication systems, the RFID (Radio Frequency Identification) system has been known. The RFID system generally includes wireless tag (also called RFID tag) and reader/writer (RW) units, in which RW units read and write data from and into wireless tags through wireless communication.
Known wireless tags are classified into a type (called an active tag) which operates through the use of the power source incorporated in the wireless tag itself and a type (called a passive tag) which operates through the use of radio wave received from RW units as driving power.
In a RFID system using passive tags, wireless tags operate the integrated circuits, such as ICs or LSIs, incorporated therein through the use of the radio signals received from RW units as driving power, and thereby carry out various processes in accordance with received radio signals (control signals). Transmission from wireless tags to RW units uses reflected wave of the received radio signal. In other words, a tag ID and results of the various processes are superimposed on the reflected wave, which is transmitted to the RW units.
RFID systems have used various frequency bandwidths. Recently, more attention has been paid to the UHF bandwidth (860 MHz through 960 MHz), which is capable of long-distance communication as compared to the 13.56-MHz and 2.45-GHz which have been put into practice. In the country of Japan, the bandwidth from 952 MHz through 954 MHz has been allocated to RFID systems.
A conventional technique related to an antenna used for wireless tags is disclosed in the Patent References 1 to 3 and a Non-Patent Reference 1 listed below.
Patent Reference 1 aims at providing a loop antenna having the enhanced antenna ability and discloses possession of a loop antenna main body which is formed of a line- or band-shaped conductive material having a form of a loop shape and which has a pair of power-supply points and a material (a parasitic element) for improving the antenna ability which material satisfies predetermined conditions.
Patent Reference 2 aims at providing a wireless tag having a configuration that enables communication at a number of frequency bandwidths, and discloses a first conductor, which has a length of about ½ wavelength and a form of a loop with opposite sides substantially parallel to each other and which is supplied with power at the center of one side of the loop, and a line-shaped second conductor disposed in vicinity of the first conductor.
Patent Reference 3 aims at providing a ring antenna with a parasitic element which antenna has improved narrow-bandwidth characteristics and an improved gain, and discloses possession of at least one basic ring antenna element and parasitic element formed of a first conductor and a second conductor which sandwich the basic ring antenna element and which are arranged in the electric-field direction of the basic ring antenna. Patent Reference 3 also discloses that the relationship 0.3×λo≦La≦0.55×λo is satisfied where the symbol La represents the length between the outer ends of the first conductor and the second conductor and the free space wavelength of the using frequency fo of the at least one basic ring antenna.
Non-Patent Reference 1 discloses a wireless tag antenna including a line-shaped (band-shaped) radiating body and a loop-shaped power-supply element (feed loop) which is arranged in the width direction of the radiating body at a distance d and which is inductively coupled to the radiating body.
Patent Reference 1: Japanese Patent Publication No. 2000-77928
Patent Reference 2: Japanese Patent Publication No. 2004-295297
Patent Reference 3: Japanese Patent Publication No. 2006-33298
Non-Patent Reference 1: H.-W. Son and C.-S. Pyo, “Design of RFID tag antennas using an inductively coupled feed”, Electronics Letters, Vol. 41, No. 18, 1, September 2005
Characteristics of matching (matching loss) between an antenna (hereinafter called a “tag antenna”) of a wireless tag and an integrated circuit such as an IC or an LSI is an important factor which determines the capacity (communication range) of the wireless tag.
The impedance (Z=R+jX) of the integrated circuit used in the wireless tag has, for example, a real part (resistance component R) of approximately dozens ohm (Ω) and imaginary part (reactance component jX) of approximately −j hundreds ohm. The tag antenna should match the impedance, that is, should establish a relationship of complex conjugate between the impedance of the tag antenna and the impedance of the integrated circuit.
A wireless tag has a matching state easily affected by an article (metal, plastic, paper, and others) to which the tag is to be affixed to or an article positioning in vicinity of the tag (i.e., easily vary the communication range and in some occasions, communication is disabled).
For this reason, there has been arisen a demand for a configuration of a wireless tag whose matching can be easily adjusted.
However, the techniques disclosed in Patent References 1 to 3 have a configuration in which an integrated circuit is directly coupled with a power-supply section of a loop-shaped antenna (hereinafter also called an “antenna pattern” and a “loop antenna”), in other words, a configuration in which the antenna pattern and the power-supply section are formed into one body. Such a configuration has an extreme difficulty in attaining (adjusting) impedance matching between the antenna pattern and the chip circuit. In particular, it is extremely difficult to control (adjust) the resistance component (R) and the reactance component (X) of the impedance (Z) independently from each other (in other words, to make it possible to attain matching with any integrated circuit having different R and/or X).
In Patent References 1 and 3, the parasitic element arranged in vicinity of the antenna pattern aims at improvement in antenna gain and at stabilization of the frequency properties of the scattering cross section, but not at impedance adjustment. In the meantime, the parasitic element (the second conductor) arranged in vicinity of the antenna pattern in Patent Reference 2 is surely for impedance adjustment, but is incapable of adjusting the resistance component (R) and the reactance component (X) independently from each other (the reference does not teach or suggest the adjustment).
On the other hand, Non-Patent Reference 1 discloses a wireless tag capable of modifying the resistance component (R) and the reactance component (X) independently from each other. In other words, according to formula (5a) of Non-Patent Reference 1, the resistance component R can be varied depending on the distance d (mutual inductance M) between a line-shaped radiating body and the loop-shaped power-supply element while according to formula (5b), the reactance component X can be varied depending on the length (Lloop) of the loop-shaped power-supply element.
However, in order to vary the resistance component R in the technique of the Non-Patent Reference 1, it is required to modify at least the distance d, that is, to modify arranged positions of the radiating body and the loop-shaped power-supply element, which may increase the size of the wireless tag under some impedances of the integrated circuit. Therefore, it is difficult to make the wireless tag small.
To a wireless tag, a protection (reinforcement) material 400 is sometimes provided as illustrated in items (1) and (2) of FIG. 16 which material covers an integrated circuit 300 for protection of the integrated circuit 300 or for reinforcement of the wireless tag. If an antenna pattern 100 and a power-supply section, with which the integrated circuit 300 is coupled, are formed into one body, one or more positions (crossing points) are generated at which the edges (ends) of the protection material 400 traverse the antenna pattern 100, and folding load concentrates on these positions. Thereby, the antenna pattern is disconnected at these positions.